The present invention relates to holding a pipe under tension sore particularly, the present invention relates to a clamp system installed on a floating support enabling a pipe to be held under tension and locked in position. The present invention also relates to a floating support including a device for connecting together and laying successive portions of an off-shore pipe from said floating support.
The present invention thus applies to holding a substantially vertical pipe in suspension, and also to any type of duct for conveying a fluid, electricity, optical signals, or information, and in particular rigid or flexible tubular pipes, umbilical cords, and electric cables. It applies in particular to off-shore pipes handled from a ship performing installation operations, in particular over very great depths, i.e. in depths of about 1000 meters (m) or more.
Drilling operations are performed from a derrick fitted with a winch and tackle for handling a string of rods. As drilling progresses, the string is lengthened by adding rods, in general in unit lengths that are multiples of 12 m, corresponding to the height available under the hook of the derrick. Said unit length is generally assembled by screwing the new length to the already-assembled string of rods that is held in suspension from the rotary table, using a clamp system of the type having self-locking wedges that leave bite marks.
In the same manner, off-shore pipes are laid in very great depth from floating supports fitted with handling means constituted by an optionally-articulated tower, possessing handling functions similar to those of a derrick. At great depths, laying is said to be xe2x80x9cJ-layingxe2x80x9d since the portion of the pipe between the level of the ship and its point of contact on the sea bed is J-shaped. During laying, successive pieces of pipe are connected end to end, and this requires the already-immersed portion of pipe to be held so that the end of this portion can be presented to the new piece of pipe, then to operate assembly tools, and then to guide the pipe so as to lay it in the sea. The use of clamps with marking wedges made of steel for holding the suspended portion of pipe suffers from the drawback of leaving traces on the outside of the pipe, and above all of damaging its anticorrosion coating. Numerous other types of clamp have been developed, based essentially on toggle-hinge systems or on cam-locking systems, with the movements of engaging and of disengaging a clamp often being driven by hydraulic actuators.
Such clamps are well adapted for loads that are small, but in very great depths, e.g. 2500 m, the weight of a pipe can exceed 500 (metric) tonnes and it is necessary to guarantee an extremely high level of safety whatever the type of pipe, during each stage of end-to-end connection, where a cycle can last for 15 minutes to 45 minutes, or indeed in the event of operations being interrupted for a variety of reasons such as an accident with the equipment or a storm, which might last for several days. If there is no need to ensure that the outer coating of the pipe is preserved undamaged, then clamps with marking wedges of the kind described above are entirely suitable for performing the function of holding the pipe, even if it presents irregularities in diameter, but if said coating is fragile, that type of clamp should not be used.
Numerous solutions have been developed to avoid damaging the outsides of pipes, but they are not adapted to extreme loads since they present the drawback of not guaranteeing positive safety in the event of slip phenomena starting, thereby running the risk of giving rise to severe accidents, not only to the equipment, but also to personnel.
The problem of the present invention is thus to be able, with a maximum level of safety and under extreme levels of loading, both to retain a pipe that presents an outside surface that has the reputation of being fragile, and to ensure that operation can be performed safely, even in the event of small dimensional variations in the outside shape of the pipe or its outer coating.
To do this, the present invention provides a clamp system installed on a floating support, enabling a pipe to be held under tension and comprising:n
a tubular element suitable for containing said pipe coaxially and secured to said floating support at the level of its assembly plane;
said tubular element containing a plurality of friction shoes;
each of said shoes being mounted on the end of a piston of a preferably-hydraulic actuator;
said actuators and shoes being disposed in a plurality of planes of said tubular element along the longitudinal axis of said tubular element, and being distributed in each of said planes around said pipe, preferably symmetrically about said longitudinal axis of the tubular element; and
said pistons moving in translation in a directions perpendicular to said longitudinal axis of said tubular element, whereby:
said friction shoes are in a clamping position against said pipe when said pistons are in an extended position, and said shoes are in a position disengaging and releasing said pipe when the pistons are in a retracted position; and
each actuator being provided with means for locking said piston in the extended position. When a hydraulic actuator is used, hydraulic pressure can be relaxed while still keeping said piston in its position clamped against said pipe.
The clamp system of the invention ensures that the pipe is retained with a high level of safety since it enables the loads to be spread over a broad zone of the pipe both within a given plane and also between different planes.
While the shoes are being clamped against the pipe, it is possible to modulate the pressure applied on the shoes so that they flatten and distribute the compression force on the pipe in a manner that is substantially uniform. In addition, it is possible to apply pressures that are different between the various planes, or indeed between the various shoes within the same plane, depending on the type of pipe and on the type of outer coating.
The tubular element preferably comprises at least two, and preferably at least four shoes in each of said planes. Said tubular element comprises at least three planes, and preferably at least nine planes. Said tubular element can have 30 or even 50 friction shoes.
The pressure with which said shoes are clamped against said pipe can be adapted plane by plane or within a given plane, actuator by actuator.
Increasing the number of planes thus makes it possible to achieve the desired retaining force, insofar as the flexible elements as constituted by the friction shoes absorb displacements of the pipe.
In the clamp of the invention, the actuator may be a mechanical actuator with a lockable drive shaft or it may be a hydraulic actuator, in particular one having a leaktight check valve provided with an accumulator using a diaphragm or a piston, in particular for compensating losses through gaskets, and of a volume that is determined as a function of expected rates of leakage for the maximum length of time the clamp might need to be in operation.
Said means for locking said piston is preferably constituted by motorized screw-driving means, said piston being secured to a nut which co-operates with a motor-driven screw so as to move in translation and lock said piston in its extended position by blocking the screw.
Each of the actuators is thus provided with a motor-driven screw-and-nut device allowing the rod to be actuated into its position where it is clamped against the pipe. The screw can be blocked either by mechanically locking the rotary element, or by means of an irreversible gearing device, or indeed by mechanically locking the drive shaft since the amount of torque required for holding it is small.
Mechanical and hydraulic actuators are known to the person skilled in the art in the fields of mechanical and hydraulic engineering and they are not described in greater detail herein.
In a preferred embodiment:
said actuator comprises a hollow actuator body secured to said tubular element constituting the clamp structure;
said piston comprises a rod having a tapped bore which co-operates with a screw so that by being unscrewed said screw moves out from said bore and its end comes into abutment against the end wall of said hollow body, thereby preventing said piston from moving when it is in said extended position, and said screw has a longitudinally fluted bore co-operating with a fluted shaft extending coaxially in said screw, whereby:
said screw is screwed in and out by motor-driven rotation of said fluted shaft; and
when the screw is screwed into said rod, said piston can move freely in translation in said direction perpendicular to said longitudinal axis of the tubular element under the effect of hydraulic pressure from its retracted position in which the piston comes into abutment against the end wall of said hollow body to its extended position, in which position the end of said screw can be brought into abutment against the end wall of said hollow body, thus preventing said piston from moving when said piston is in its extended position clamped against said pipe.
Said friction shoes are preferably constituted by an elastomer layer mounted on a rigid shoe support.
In an advantageous embodiment, an elastic flexible hinge is incorporated in the shoe support or between said shoe support and the piston, said hinge transmitting stresses at the bearing surface of said friction shoe with said pipe due to movement of said pipe in translation relative to said tubular element, said translation comprising a longitudinal component along the axis of said pipe and a transverse component in a direction perpendicular to said axis of said pipe.
In particular, the elastic flexible hinge is constituted by a laminated abutment made up of a plurality of elastomer layers separated from one another by reinforcements of rigid material.
Advantageously, said laminated abutment is disposed in inclined manner, preferably at 5xc2x0 to 25xc2x0, and more preferably at 10xc2x0 to 20xc2x0, relative to the longitudinal axis of said tubular element so that when the pipe tends to slide along said longitudinal axis, in particular downwards, the force clamping the shoe against the pipe is increased because of a self-wedging effect. This laminated abutment in which the elastomer layers are inclined constitutes a self-wedging or self-engaging device that prevents the pipe from sliding under the effect of a load in the longitudinal direction of the axis of said pipe, insofar as said shoe maintains a surface in contact with said pipe.
In a particular embodiment, said laminated abutment is in the form of a portion of a frustoconical surface of revolution with its smaller circular base situated in particular at its bottom end so as to prevent downward sliding of said pipe, the generator line of said portion of a surface of revolution being inclined, preferably at 5xc2x0 to 25xc2x0, and more preferably at 10xc2x0 to 20xc2x0, relative to the longitudinal axis of said tubular element.
In an embodiment, said shoe supports are mounted on respective ball-and-socket joints enabling said supports to pivot and preferably constituted by laminated abutments of spherical type.
In an advantageous embodiment, said friction shoe is not disposed symmetrically about the plane perpendicular to the longitudinal axis comprising the axis of said actuator, said shoe being offset downwards relative to said axis of said actuator. Thus, the stress applied to the pipe by the actuator is greater near the top than it is near the bottom of the shoe. In the event of slip starting, the compression force increases, thereby giving rise to creep in the outer wall of the pipe. Said creep is greater near the top where the stress is highest, thereby increasing initiation of the self-wedging or self-engaging effect of the device.
Advantageously, said friction shoes are constituted by a plurality of blocks of elastomer material having marking-effect metal jaws interposed between them and secured to the shoe support, said jaws being set back from the bearing surface constituted by said blocks. Thus, in normal operation, the entire load is transferred from the shoe to the pipe via said blocks of synthetic material. However, in the event of the radial force becoming excessive, creep of the outer wall of the pipe puts a portion of said outer wall into contact with said metal elements that produce a marking effect, thus providing an ultimate degree of safety under extreme conditions, while nevertheless minimizing the traces created by the metal jaws on the coating of the pipe.
In an advantageous embodiment, a beam is disposed between the piston and the shoe substantially parallel to the longitudinal axis of the tubular element, said beam being connected to a beam support via a hinge whose axis is situated at the bottom of the beam, said support element for the beam itself being secured to the tubular element. This embodiment makes it possible to reduce considerably the bending forces that are applied on the axis of the actuator, since vertical forces are taken up by said beam.
The present invention also provides apparatus for connecting together and laying successive lengths of an off-shore pipe from said floating support. According to the invention, said apparatus includes a clamp system in order to lock and hold in suspension a first portion of pipes in a vertical or an inclined position, ready for assembly with a second portion of pipe prior to said first portion of off-shore pipe being fully immersed.